1. Field of the Invention
Methods and apparatuses consistent with exemplary embodiments relate to an image forming apparatus and a method of forming an image thereof, and more particularly, to an image forming apparatus which generates a horizontal synchronization (sync) signal to compensate for a deviation in reflective surfaces of a polygon mirror in order to prevent deterioration of printing quality, and a method of forming an image thereof.
2. Description of the Related Art
It is common that image forming apparatuses using an electrophotographic method, such as laser printers, copiers, multi-function peripherals, and facsimile machines include a laser scanning unit. The image forming apparatus forms an electrostatic latent image on a surface of a photosensitive medium using laser beams output from the laser scanning unit, transfers the electrostatic latent image to paper, and prints a desired image.
Since the image forming apparatus should output a video signal (or image) to be printed to the photosensitive medium on time, the image forming apparatus is required to generate a horizontal sync signal to control an outputting time of the video signal without error.
Therefore, the conventional image forming apparatus is equipped with the same number of beam detectors as light sources provided in the laser scanning unit in order to detect beams output from the plurality of light sources and reflected, and generates a horizontal sync signal with reference to a beam detection signal of each light source.
However, there has been an attempt to use a single beam detector regardless of the number of light sources for the purpose of saving the cost of materials.
Referring to FIG. 1, the image forming apparatus generates two horizontal sync signals (Hsync) by applying a predetermined time offset according to a beam detection signal (BD) output from a single beam detector. In this case, video data signals (VDO Data) are generated with reference to the horizontal sync signals (Hsync), and, while the video data signals (VDO Data) are generated, beams projected from the light sources enter a surface of a photosensitive medium through a polygon mirror and a reflective mirror, thereby forming a latent image.
In FIG. 1, it is assumed that the polygon mirror is ideally manufactured. That is, since there is no deviation in the reflective surfaces of the polygon mirror, horizontal sync signals (Hsync (M,Y)), which generate video data signals (M, Y VDO Data) using beams that are emitted from light sources but are not directly detected by the beam detector, can be easily estimated using the beam detection signal (BD) detected by the beam detector.
If there is no deviation in the reflective surfaces of the polygon mirror as described above, the horizontal sync signals (Hsync (M, Y)) for the light sources that have no beam detector can be generated, predicting a starting point at which the video data signals (M, Y VDO Data) are generated exactly, using a length of the reflective surface of the polygon mirror and a rotation phase difference of the polygon mirror.
However, if there is a deviation in the reflective surfaces of the polygon mirror, the incoming beam detection signal (BD) has a different period according to each reflective surface of the polygon mirror, and thus, it is impossible to generate an exact horizontal sync signal. As a result, image quality deteriorates as shown in view (b) of FIG. 8.
As described above, since image quality may deteriorate if there is a deviation in the reflective surfaces of the polygon motor, the related-method using two beam detectors and detecting a beam detection signal for every light source should be used, or a strict criterion for judging defectiveness of the polygon motor should be established in order to prevent a deviation in the reflective surfaces. However, there is a problem in that these methods result in increased material costs.